The present invention relates to the detection of chemical substances of known masses M.sub.i using an ionization, dissociation, filtration and detection apparatus successively making it possible to produce ions of the substances to be detected, their dissociation by impacts on neutral gas molecules and the seeking of dissociation fragments of known masses m.sub.i of the substances of masses M.sub.i to be identified.
Detection apparatuses and processes of this type are known and are in particular described in French patent application 8715212 of Nov. 3, 1987 in the name of the Applicant. This document teaches with the aid of an apparatus incorporating an ion source, a dissociation case, an electrostatic analyzer, a magnetic analyzer and an ion detector, a method which makes it possible to determine the presence in an atmosphere to be monitored of a chemical substance of known mass M on the basis of the examination of the dissociation fragments of also known mass m.sub.1, m.sub.2, . . . m.sub.p. The method fundamentally consists of identifying a sought substance of mass M by revealing the existence in the atmosphere to be monitored of a sufficiently large number of its dissociation fragments and said result is achieved by acting both on the extraction voltage for the ions at the outlet from the ion source and on the energy filtration level of the electrostatic analyzer.